Human and Primate Evolution
Human and Primate Evolution
The diversification and cultural development of humans occurred only in the last few million years, but the species has a much longer evolutionary background. Humans are primates, related to apes, monkeys, and lemurs, and many of the unique characteristics of the species are a result of the social and ecological interactions of our ancient primate ancestors. Human evolution built upon general primate adaptations by elaborating several major innovations, such as upright walking, tool use, culture, and, ultimately, language.
Since the late twentieth century, there has been an explosion of genetic information about humans and other primates. The Human Genome Project and subsequent projects exploring the genomes of related primates have made it possible to examine the genetic changes that underlie human and primate anatomy and behavior. This has led to a reevaluation of many old hypotheses concerning primate and human evolution, as well as the formulation of new ones, most notably, the recognition that humans and chimpanzees are sister taxa. Anthropologists can now employ a combination of genetic information and evidence of fossil form (morphology) to test hypotheses about human evolution.
Molecular comparisons of living primates suggest that the last time they all shared a common ancestor was sometime during the Late Cretaceous period, around 80 million years ago (Ma). The first primates would have been animals similar in form and adaptation to living tree shrews, which are small arboreal insectivores. The earliest fossil evidence of primates is from the Paleocene epoch, between 65 and 55 Ma.
The initial diversification of the primates may have been a case of coevolution with flowering plant species, for whom modern primates, bats, and plant-eating birds are important pollinators and seed dispersers. Today, nearly all primates retain a generalized, broad diet made up of a balance of fruits, leaves, plant gums, and insects or meat, with some primate lineages specializing to some extent on one or another of these sources. Early primates left humans an anatomical legacy: arboreal adaptations such as grasping hands, fingernails, and binocular vision. They also left a legacy of sociality, as most living primates form long-term social bonds that include mutual grooming.
The prosimian primates include living and fossil lemurs, lorises, and tarsiers. Lemurs live today only on Madagascar; East African bushbabies and South Asian lorises are their close relatives. Tarsiers now live on Southeast Asian islands. In the Eocene and Oligocene (c. 50–30 Ma), lemur-like adapid primates and tarsier-like omomyid primates were broadly distributed through the forests of North America, Europe, Africa, and Asia. The Eocene was the warmest period of the last 65 million years, and subtropical forest habitat suitable for primates covered areas as far north as Wyoming and France.
Monkeys, apes, and humans are grouped together as anthropoid primates. Living anthropoids share a number of features attributable to their common ancestry. These primates tend to invest more resources and time into their offspring, with longer developmental times and more extensive brain growth. These features allow more sophisticated social behaviors, with stable social groups that effectively share information. Nearly all anthropoids give birth to one offspring at a time, and females have a single-chambered uterus to enable longer gestations and larger fetal size.
Early anthropoids appeared during the Late Eocene (c. 40 Ma). Anthropoids like Aegyptopithecus from Fayum, Egypt, had skeletons like living monkeys in most respects, but with relatively smaller brains. Genetic comparisons of living humans and monkeys show that genes expressed in brain development have evolved rapidly during the last 30 million years, reflecting the recent evolution of cognitive functions in anthropoids (see Dorus et al. 2004).
Humans and apes are hominoids, and they diverged from the cercopithecoids (Old World monkeys) around 30 Ma. The first hominoids were similar to earlier anthropoids. They were arboreal quadrupeds, unlike living apes, which have long arms for suspending their bodies beneath branches. The teeth of early apes were like those of earlier primates. Proconsul was an important fossil hominoid lineage in Africa from 24 to 15 Ma, with several species covering a range of size from monkeys like macaques up to chimpanzee-sized or larger. The diversity of these apes covered many of the size and diet niches now occupied by cercopithecoids. At the same time, a gorilla-sized African ape called Morotopithecus appears to have had a suspensory locomotor pattern. Genetic evidence suggests that the most diverse lineages of living apes, the gibbons and siamangs, diverged from the ancestors of the great apes sometime around 18 Ma.
A dispersal of hominoids into Eurasia during the Middle Miocene may have included the ancestors of living great apes. Several apes, including Ankarapithecus and Pierolapithecus, were relatively small apes, with arms suited to suspending their weight beneath branches like living gibbons. These apes divided into an Asian lineage, ancestral to living orangutans, and a European-African lineage, ancestral to humans, chimpanzees and gorillas. The number of genetic differences between living species can be used to estimate the length of time since they last shared a common ancestor, called their divergence time. For the Asian and European/African ape lineages, this divergence occurred around 13 Ma. An orangutan-like ape called Sivapithecus existed in South Asia by 12 Ma.
Toward the end of the Miocene, ape diversity declined. South Asian and European apes ultimately became extinct, coincident with climate changes that increased seasonal temperature and rainfall variations and reduced the area of forests. These climatic shifts favored the rise of the cercopithecoid (Old World) monkeys, whose geographic range increased during the Upper Miocene and Pliocene to include Europe and East Asia by the Early Pliocene (c. 5 Ma).
Both humans and the living great apes are survivors of these extinctions. Despite being limited to small geographic ranges in the tropical forests of Africa and Indonesia, great apes have substantial adaptive and genetic diversity. For example, the genetic differences between Sumatran and Bornean orangutans exceed those between many other primate species. Likewise, chimpanzees and gorillas retain behaviorally and genetically distinct sub-species across their African ranges. These primates depend on different foods, strategies for finding food, and styles of communication in different parts of Africa.
Unlike the other African apes, early hominids are exceptionally well preserved in the fossil record almost immediately after their origin. Three hominid species have been found dating to the Late Miocene: Sahelanthropus tchadensis (7 Ma) in Chad, Orrorin tugenensis (6 Ma) in Kenya, and Ardipithecus kadabba (5.5 Ma) in Ethiopia. Each is represented by a fragmentary sample that presents some evidence of bipedal locomotion or upright posture (e.g., the proximal femur of Orrorin, the cranial base of Sahelanthropus, and the foot of Ardipithecus). The dental remains of these genera are very similar and, except for their smaller canines, within the range of other Late Miocene apes (see Haile-Selassie et al. 2004; Wolpoff et al. 2006).
A rich record of early hominids exists from sites in eastern, southern and central-western Africa. These remains date from as far back as nearly 4 Ma. The famous “Lucy” skeleton, found in 1974 in Hadar, Ethiopia, represents the species Australopithecus afarensis, and an even more complete skeleton was found in Dikika, Ethiopia in 2001. Hundreds of other fossil fragments from Ethiopia, Kenya, and Tanzania also belong to this species, which lived between 3.8 and 2.9 Ma. An additional large sample of hominids found in South Africa and dating to between 3.0 and 2.5 Ma represents Australopithecus africanus. This was the first of the early hominid species to be discovered. It was first identified by the South African anatomist Raymond Dart in 1924. From the name Australopithecus, these early hominids are often called “australopithecines.”
These samples confirm the importance of bipedal locomotion to the early hominid lineage. The shape of the pelvis, knees, and feet had evolved to a human-like form that precluded efficient quadrupedal locomotion. Footprint trails from 3.5 Ma found in Laetoli, Tanzania, also demonstrate their human-like bipedality. Several pieces of evidence suggest that these australopithecines retained an adaptation to climbing. In particular, this may explain their short legs, small body sizes, powerful arm bones, and curved hand bones. Their small size stands out as a contrast to recent humans, as they averaged only around 1.2 meters in height and 35 to kilograms in mass.
Aside from bipedality, the other major anatomical pattern of early hominids involved dentition. Australopithecines had large molar and premolar teeth compared to living and fossil apes and humans. These teeth were low-crowned and had thick enamel, apparently adapted to a diet of grinding hard foods such as seeds. Isotopic evidence suggests that their diet was varied, with the main difference from other primates being a high consumption of plants with a C4 photosynthetic cycle—including grasses and some sedges (see Sponheimer et al. 2005). As primates cannot digest grass, it has been suggested that this may represent the consumption of grass seeds, termites, and other grass-consuming animals (see Peters and Vogel 2005). Contrasting with their large molar teeth, early hominids had small canine teeth, which may hint at a reduction in male competition or a shift from threatening displays with the canine teeth to other kinds of displays, such as vocalizations or weaponry.
A later group of australopithecines greatly emphasized the adaptation to large grinding teeth. These “robust” australopithecines had molar and premolar teeth with as much as four times the area of present-day humans, together with immense jawbones and jaw muscles. Their diet presumably included a higher percentage of hard, brittle foods, which may have been increasingly important during the drier climates of the Late Pliocene. These were the last of the australopithecines to become extinct, a little less than 1.5 Ma.
Alongside the robust australopithecines lived the earliest members of our own genus, Homo. Early Homo can be distinguished from contemporary australopithecines by its smaller molar teeth (although still larger than living people) and larger brain size. The transition to large brains and smaller teeth was accompanied by an increased dietary reliance on meat. Because of its high caloric and protein content, meat requires fewer digestive resources and can fuel more substantial brain growth. Primates with high-energy diets tend to have smaller guts, which also allows a higher proportion of metabolic resources to be allocated to brain tissue (see Milton 2003; Aiello and Wheeler 1995).
The archaeological record provides further evidence for a dietary shift, with the earliest-known stone tools occurring in Ethiopia about 2.6 Ma. Many primates are able to manipulate objects as tools, and wild chimpanzees have traditions involving the use of stones to crack nuts and shaping simple wooden spears or probing sticks. It seems probable that early hominids also shared these abilities, but they left no archaeological trace. The earliest flaked stone tools were used to cut flesh off animal bones and break into bones for marrow.
These early toolmaking hominids existed in regions with more extensive and seasonally arid grasslands, and they are found together with the robust australopithecines. Both fossil and archaeological evidence of Homo remain rare before 2 Ma, but after this time numerous fossils of a small-bodied, large-brained hominid species called Homo habilis have been found. Homo habilis is the first species to show evidence in the wrist and hand of toolmaking adaptations, and the traces of brain anatomy preserved on its endocast suggest a more advanced planning ability than in earlier hominids (see Holloway 1996).
A second species of early Homo, called Homo erectus had larger bodies and taller stature—an average of 1.6 to 1.8 meters compared to earlier hominids at 1.0 to 1.4 meters. With its longer legs and larger brain size, Homo erectus was adapted to the use of larger home ranges and more patchily distributed, high-energy food resources. The differences in size between males and females in this species, sexual dimorphism, were in the range of recent humans, possibly reflecting more human-like social interactions than in earlier hominids, including greater cooperation and food sharing.
The use of more open territory and larger home ranges may have enabled Homo erectus to colonize Eurasia. A series of fossils and archaeological remains from Dmanisi, Republic of Georgia, dates to about 1.8 Ma. Hominids also reached Java around this time, and indeed the first fossil specimens of Homo erectus to be found were discovered on Java by the Dutch colonial physician and scientist Eugene Dubois in 1891.
The populations of Homo erectus in Africa and East Asia developed some regional differentiation relatively early in their existence. The form of the cranium, the thickness and shape of the brow ridge, the size of neck muscle attachments, and other details overlap between regions but differ substantially on average. Also, the Dmanisi Homo erectus skeletons appear to have been smaller than those in Africa. Some researchers view these features as evidence that Homo was divided into different species in different parts of the world. Others consider these morphological differences to be analogous to features distinguishing human populations today (see Asfaw et al. 2002).
Homo erectus had reached China by 1.2 Ma, but hominids entered Europe later, after 1 Ma, and possibly as late as 800,000 years ago. Shortly after this time, fossils in Africa show a loss of some of the diagnostic cranial traits of Homo erectus, and, with a few exceptions, early European skulls never had them. The African and early European remains are often referred to as “archaic” members of our own species, Homo sapiens, or else by another species name, Homo heidelbergensis. It is not clear whether the anatomical evolution was accompanied by biological speciation, or whether it represents an increase in brain size and consequent changes in cranial morphology within a single evolving species. In either case, early European hominids also had morphological features distinguishing them from other regions, including a projecting face and nose and large sinuses. One of the most important sites of the last million years is Sima de los Huesos, Ataperca, Spain, at which the partial skeletal remains of more than 25 individuals, from around 300,000 years ago, have been found.
The emergence of regional morphological variants was one trend during the Pleistocene, and it was joined by other trends in common across different regions. The most important was a gradual increase in brain size. The earliest Homo erectus specimens had endocranial volumes averaging around 750 milliliters; these increased to an average of 1,400 milliliters by 50,000 years ago. This increase is evident everywhere ancient humans lived, including Africa, Asia, and Europe. It is logical to assume that brain size increased because of new cognitive abilities. Brains are energetically expensive, and the metabolic cost of an increase in brain tissue must be redeemed by more food acquisition or reproduction.
The archaeological record provides additional evidence about cognitive evolution. Stone tools gradually became more sophisticated over the Pleistocene. First, the development of bifacially flaked handaxes and cleavers in Acheulean industry shows that hominids could learn and replicate standardized, symmetrical forms by 1.5 Ma. Later, tools became more standardized, raw materials were obtained across longer distances, and techniques were shared across wider areas. These changes may reflect either more widespread contacts between cultural traditions or more efficient transfer of information. Finally, by 300,000 years ago, humans had mastered prepared-core toolmaking techniques, which required information transfer, not only about finished tool form but also about procedure. After this time, the technological properties of different human cultures began to diversify yet further, with industries changing more rapidly and occupying smaller areas. The fragmentation and acceleration of change in material culture would continue over the last 50,000 years as the complexity of culture and behavior increased further.
It is likely that the behavioral complexity after 300,000 years ago required some capacity for spoken language. Because people learn and coordinate their activities by talking to each other, language is a fundamental basis for human culture and behavior. But there is very little anatomical evidence relating to the evolution of language, for the necessary structures (e.g., tongue, larynx, brain) do not fossilize. Still, a few hints exist. At least one Homo habilis skull includes a marked enlargement of Broca’s area in the frontal cortex, a brain structure important to planning complex activities and carrying out speech in living humans (see Holloway 1996). The hyoid bone, a small bone in the throat that supports the larynx, rarely fossilizes, but two hyoids from Sima de los Huesos and one from Kebara, Israel, have been found. These hyoids are essentially human-like in shape, in contrast to a preserved hyoid from Australopithecus afarensis, which is ape-like. Finally, at least one gene related to language, FoxP2, shows evidence of strong selection within the past 200,000 years (see Enard et al. 2002). Together, these hints suggest a long evolution of language from early, simple communication to the fully human language of today.
The most well-known group of ancient humans is the Neanderthals (or Neandertals), inhabitants of Europe and parts of West Asia between 200,000 and 30,000 years ago. The Neanderthals were specialists in hunting large game, with sites dominated by the bones of bison, horse, and red deer. Isotopic evidence suggests that their diet included a very high proportion of meat (see Bocherens et al. 2005). Early humans, including Neanderthals, had short lives compared to recent humans, including recent hunter-gatherers. They also had a very high rate of traumatic injuries. These factors may be attributable to their reliance on close-contact hunting of large animals using thrusting spears. With powerful long bones and muscular necks, the Neanderthals were highly adapted to this strenuous lifestyle.
The high mortality and risks of early human lifestyles had demographic consequences. Archaic humans maintained low population densities and low total numbers for thousands of generations. In contrast, recent humans have exploded exponentially in numbers. This rapid growth has been possible with a relatively small per-generation rate of increase, emphasizing that the reproductive potential of early humans must have been balanced by higher mortality. The risks of ancient lives may also be illustrated by the occurrence of cannibalism, both by Neanderthals and other archaic peoples.
Genetic evidence taken directly from Neanderthal skeletal remains has been recovered. Some of the diversity of ancient Neanderthals is evidenced by their mitochondrial DNA (mtDNA), which share a common ancestor with the mtDNA of living humans between 300,000 and 700,000 years ago (see Serre et al. 2004). No sequences like the Neanderthal mtDNA have been found in any living people, however, suggesting that at least this genetic element did not form part of the ancestry of present-day humans. The relationships concerning the rest of the genome are somewhat more complicated. The initial phase of the Neanderthal Genome Project found possible evidence for Neanderthal-human inter-breeding, with Neanderthals differing only slightly more from humans than a random pair of humans do from each other. Other genetic evidence from recent people also suggests that genes from archaic humans may have entered recent human populations by interbreeding.
Neanderthals and other early humans were absorbed or displaced by the emergence of modern humans. This event may reflect the simple technology of earlier peoples and the more effective collection strategies of moderns, and it therefore may have been a primarily cultural transition with anatomical and behavioral consequences. Alternatively, there may have been a cognitive revolution between the earlier archaic and later modern humans. In any event, the later historical elaboration of human cultures and diversity would not have been possible without the evolutionary history of Pleistocene and earlier hominids. The anatomical and behavioral adaptations of our ancestors were the building blocks of the current human world.
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